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This Week on Galileo?
Today on Galileo
Monday, October 15, 2001
DOY 2001/288

The Io 32 Encounter Begins

Well, the preliminaries are out of the way, and today is the busiest on Galileo's agenda for this encounter, number 32 in the series, with an evening flyby of the volcanic satellite Io.

The day begins at 1:12 a.m. PDT [see note 1] with the Photopolarimeter Radiometer instrument (PPR). The instrument turns its power on and performs a calibration sequence to prepare for a full day of science observations. Then at 1:29 a.m., the instrument turns its gaze on Jupiter for a 5-hour-long series of observations of the turbulent atmosphere of the giant planet. These measurements blanket the northern polar region studying an area populated by vortex-like storms. An additional observation at 4:42 a.m. collects data from the White Oval in the southern hemisphere. This particular storm is the lingering result of a merger of several such storms over the past few years, and has been the subject of repeated study by Galileo's science instruments. Then at 8:01 a.m. PPR begins viewing Io, mapping the temperatures of the surface of the dark side of the satellite.

At 9:11 a.m. the Fields and Particles suite of instruments begins a 2-hour recorded measurement of a portion of Jupiter's magnetosphere near the Io Torus known as the Ramp region. The torus is a doughnut-shaped area of increased radiation and particle density that nearly coincides with the orbit of Io. The Ramp is the transition between the background magnetosphere and the torus. It is a region where the ion density and temperature of the environment increase sharply, and that has not been fully explored. The instruments participating in this observation are the Heavy Ion Counter (HIC), the Energetic Particle Detector (EPD), the Magnetometer (MAG), the Plasma Subsystem (PLS), and the Plasma Wave Subsystem (PWS).

At 10:58 a.m. the spacecraft enters the shadow of Jupiter, spending nearly two hours in the dark as it continues its observations. The on-board control software is configured to ignore the fact that the Sun can't be seen. At 11:12 a.m. Galileo reaches its closest point to the largest of Jupiter's satellites, Ganymede. But at over 1.4 million kilometers (890,000 miles) distance, it, like Callisto 3.5 hours later, is too far away to present a tempting observation target.

At 11:58 a.m., an experiment conducted by the Radio Science Team begins. This measurement takes advantage of the fact that the path of Galileo takes it behind Jupiter as seen from Earth. As the radio signal sent from the spacecraft appears to pass deeper and deeper into Jupiter's atmosphere, it will be attenuated and refracted by the gases, and this will yield information about electron density in the different layers of Jupiter's ionosphere. Eventually, the atmosphere becomes too dense, and the signal disappears altogether. This will occur at 12:23 p.m. At 2:09 p.m., the signal will re-appear as the spacecraft passes out from behind the planet, and the measurement sequence will be repeated in reverse as the occultation ends. The Radio Science experiment ends at 3:41 p.m.

Even though the spacecraft is out of sight of Earth, science observations can continue, as the data are placed on the on-board tape recorder for later playback. Between 1 and 2 p.m., PPR again collects data for a thermal map of Io's dark hemisphere.

The Solid State Imaging camera (SSI) next enters the picture, as it were, at 2:24 p.m., snapping an image of Io with the giant volcano Loki appearing on the limb. This will be our highest resolution look yet at this region of Io. As we will be looking at the area from the side, we may get some insight into the local topography, measuring relative heights of the features.

At 2:33 p.m. PPR captures an hour-long high-resolution temperature map of the caldera, or volcanic crater named Colchis in Io's northern hemisphere. This observation occurs while the feature is in the dark, and will measure the intrinsic temperatures of the surface features, without being complicated by reflected sunlight.

At 3:32 p.m. PPR performs another hour-long temperature map on Io that covers the volcanic features Loki, Daedalus, and Amatarasu, in the northern hemisphere. Then another hour's observation covers the Babbar Patera in the southern hemisphere, followed by a half hour look at Ra Patera, also in the southern hemisphere.

While PPR is viewing the surface of Io, the Fields and Particles instruments are recording continuously for the 2.5 hours. At this point in the orbit, Galileo is passing through an expected "quiet" portion of the Io Torus. The instruments will be measuring the electromagnetic and particle interactions in the region, contributing to an understanding of particle pickup processes near Io, and of thermal and non-thermal plasma interactions.

During this time as well, at 4:33 p.m. the spacecraft reaches its closest point to Jupiter, at a distance of 4.8 Jupiter radii (343,000 kilometers or 213,000 miles) above the cloud-tops, slightly less than the distance from the Earth to our Moon.

At 6:04 p.m. the Near Infrared Mapping Spectrometer (NIMS) enters the fray, mapping the temperatures of the Pele volcano while that feature is in the dark. This is followed at 6:10 p.m. by a PPR observation of the volcano Loki and its environment. At 6:35 p.m. the focus on Loki continues with 10 minutes of observation by NIMS.

At 6:48 p.m. SSI shutters 7 images of the Pele volcano, which is in darkness, looking for hot, glowing material to provide an estimate of the temperatures in the region. These pictures will allow scientists to distinguish features as small as 30 meters (98 feet) across, and will cover more territory than similar pictures taken during our 24th orbit and flyby of Io. NIMS follows on with a 6-minute temperature observation of the Pele region.

At 6:58 p.m. PPR looks straight down at Io as the spacecraft flies by, and the instrument will measure temperatures of the swath of the satellite that flows past its field of view for 10 minutes. During this observation, at 7:04:20 p.m., Galileo reaches its closest point to Io, at a distance of 181 kilometers (112 miles). At this time the spacecraft is traveling at 7.72 kilometers per second (17,270 miles per hour) relative to the satellite. The latitude over which Galileo is flying is 78.6 degrees south, which is equivalent to flying over Little America in Antarctica on Earth.

At 7:08 p.m. SSI provides a high-resolution look at the Telgonus Scarp region, a cliff-like feature, looking for evidence of sapping or other erosion mechanisms. This observation recovers a similar observation planned on our 25th orbit in November 1999 which was lost when the spacecraft experienced a fault and entered a safe state.

At 7:10 p.m NIMS performs a high-resolution thermal map of the southern polar region of Io in order to map the distribution of sulfur dioxide.

At 7:14 p.m. SSI turns its attention to high-resolution images of lava channels in the Emakong region, broader context images of the Telgonus Scarp region previously viewed a scant 8 minutes earlier (we're flying away from Io at a pretty good clip, now!), and a mountainous volcano named Tohil captured near the terminator, or day-night boundary. This last viewing geometry provides good shadows to determine relative heights of features.

At 7:23 p.m. NIMS spends 10 minutes mapping temperatures and sulfur dioxide distribution around the Emakong caldera. Then both SSI and NIMS view an active caldera named Tupan Patera, with SSI providing a 3-color image of the site.

At 7:49 p.m. SSI collects a medium-resolution view of the Tvashtar volcano. This site was the focus of activity during the last flyby in August, when the trajectory of Galileo carried it nearly directly overhead of the feature at close range. Unfortunately, at that time, electronic problems with the camera prevented the return of any images near closest approach. This will be the first medium-resolution look at the feature since the large plume eruption late last year.

Two minutes later, NIMS views the Chaac caldera for thermal and sulfur dioxide mapping for 10 minutes. Then SSI takes a medium-resolution (500 meters per pixel) picture of the Gish Bar region. With this picture we can look for changes in the area since last seen in October 1999, on Galileo's 24th orbit of Jupiter. We will also be looking at a large apparent fracture to the west of Gish Bar to determine its origin.

Just after 8 p.m. NIMS performs a 15-minute measurement of a new hot spot in the northern hemisphere that was discovered on our last flyby in August. As the current sequence of commands was already being developed during that flyby, this observation represents a last-minute change to our viewing plan.

At 8:20 p.m. SSI views the terminator region of Io, which encompasses the Mycenae region, Tohil Mons and Patera, Culann Patera, Zamama, Volund, and several other poorly understood but intriguing sites. With the Sun low in the sky over these features, longer shadows should provide scientists with information about the relative heights of the features.

At 8:28 p.m. PPR returns to the observing plate with a scan along the equator of Io, providing temperature data, followed by a map of a small portion of Io's disk just south of the equator, this time looking at the polarization of light reflected from the surface. This data will provide information about the fine-scale structure of the surface materials.

At 8:54 p.m. NIMS begins an hour-long map from pole to pole of most of Io's sunlit surface, mapping the locations of hot spots and sulfur dioxide deposits. PPR follows at 10 p.m. with a 30-minute map focusing on the northern latitudes of Io.

At 10:34 p.m. SSI snaps an image of the small inner satellite Amalthea, with a view of the crater Pan on its surface. Current plans for Galileo include flying within about 500 kilometers (310 miles) of this small moon in November 2002.

At 10:36 p.m. PPR wraps up this very full day's observations with a full-disk polarimetry map of the surface of Io, which takes us to midnight.

Now, you may be left with the feeling of "What's left to do?" Come back tomorrow and see! We're not done yet!

Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 41 minutes to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time.

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